In Japanese Patent No. 4084248, brake control for a vehicle is described which sets a target slip ratio of a front outside wheel to a value higher than a normal value and controls the brake force at the front outside wheel based on the target slip ratio when a vehicle is under an avoidance maneuver operation in emergency, wherein the target slip ratio is higher than that in a spin control which works when a friction coefficient of a surface of a road (hereinafter referred to as μ) is high. More specifically, a correction coefficient is set to a higher value in order to correct the target slip ratio to a higher value when an inertia moment corresponding to an acceleration in the lateral direction of the vehicle (hereinafter referred to as a lateral acceleration) becomes larger. When the vehicle brake control under an avoidance maneuver operation starts, in order to suppress change in the W/C pressure and thereby prevent the vehicle from rolling and vibrating, duty cycles of boosting and depressurizing of the W/C pressure are set by using a map in which an increase rate and a decrease rate of the W/C pressures are suppressed compared to normal.
FIG. 12 is a graph showing a relation between the slip ratio and the lateral acceleration of the front outside wheel. FIG. 13 is a graph showing a relation between the lateral acceleration and a roll angle of the vehicle.
As shown in FIG. 12, the lateral acceleration reaches its peak value when the slip ratio becomes a certain value (approximately 5% in FIG. 12) and decreases as the slip ratio becomes larger beyond the certain value. As shown in a line 31 in FIG. 13, the lateral acceleration is proportional to the roll angle until the vehicle comes into a state where there is a possibility of the lateral overturn. Therefore, the lateral acceleration can be a parameter indicating the roll angle, which means that the roll angle becomes large as the lateral acceleration reaches its peak value. A region 32 is a region in which the lateral overturn is highly possible.
If the target slip ratio for the front outside wheel is set to a high value as in Japanese Patent No. 4084248, the target slip ratio becomes larger than a value of the target slip ratio at which the lateral acceleration reaches at its peak value. Therefore, the lateral acceleration can be suppressed if the actual slip ratio is controlled so that it becomes equal to the target slip ratio. FIG. 9 also shows an example of an anti-lateral overturn control in which the lateral acceleration decreases from a region 33 to 34 and a wheel such as front outside wheel accordingly slips. In addition, since increase or decrease of the W/C pressure changes the slip ratio and the lateral acceleration, restriction on the increase or decrease of the W/C pressure can decrease the change in the lateral acceleration and prevent vibration in rolling motion of the vehicle.
However, since the W/C pressure is always restricted during the avoidance maneuver operation in emergency in Japanese Patent No. 4084248, steerability of the vehicle deteriorates.